U.S. Pat. Nos. 4,711,128 and 4,663,972 describe a resiliently suspended seismic mass and plates disposed to fit into one another to form a capacitor or rather a capacitive accelerometer sensor which are made of monocrystalline material, in particular of quartz or of monocrystalline silicon. To manufacture this type of structure, trenches are initially produced in an epitaxy in the customary fashion by means of an anisotropic etching, and the so formed structures are then dissolved from the substrate by means of isotropic undercutting.
The disadvantage of this arrangement is that during the isotropic undercutting, the material is removed from both sides, essentially in a semicylindrical form, as shown in FIG. 2. This means that when a plate of a defined width is undercut, the height of the etch-out essentially corresponds to this width. The result is that when an epitaxy has a minimal height of, for example, 10 .mu.m, it is only possible to undercut very narrow structures. It is, therefore, difficult to produce a large enough seismic mass, given this width limitation. A further disadvantage lies in that in the case of a realization in monocrystalline silicon, the contacting and the passivation of the required sensor supply leads are problematic.
PCT Patent Publication No. WO 92/03740 describes an accelerometer sensor produced using means of surface micromechanics, in which the capacitor structures and the suspension segments, in particular, consist of polycrystalline silicon.
The disadvantage of a polycrystalline material lies in the restricted activating capacity of dopants. Especially in the case of sensors in the low-acceleration range, the suspension segments for the seismic mass must have a very long and thin design. However, to achieve high conductivity, a high level of doping is required. The component of dopant material, which is introduced into the grain boundaries of the polycrystalline material and, which consequently, is not electrically activated, can contribute to an increased compressive stress and, given a concentration gradient, also to a voltage gradient. Laying bare the structures causes the suspension segments that are held on two sides to be raised and the capacitor plates that are retained on one side to be curved. Both effects limit the maximum attainable size and conductance of the structure. Moreover, the linear deformation and the compressive stress in the suspension segments can cause the seismic mass to assume various preferred states which is undesirable and adversely affects free mobility.